I must respectfully disagree with John and Joel: compression and carbon buildup on the valves may affect how well the fuel-air mixture burns once it is sparked, but has nothing to do with the presence or absence of a spark. The question says, "no spark": gotta be a bad gap, a bad coil, a shorted wire, or (most likely) a grounded killwire. (I've read that some B&S engines have points, too: if yours does, check 'em.)
Tim, June 2011
